Add new haproxy section

Since https://github.com/openstack/neutron/commit/bf8949defc24af3636da417442469e7d7194c7b3
got merged, we have now a new section in
LBaaS configuration file: [haproxy].

This patch aims to add this section and also to fix user_group flag to
be part of this new section.

Change-Id: I6c13e550a7033daa8ae6164b0437dfbc16a07666
Closes-bug: #1263952
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Add new haproxy section"